An electrolyte solution for lithium secondary batteries is generally comprised of a combination of cyclic carbonate and linear chain carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), gamma-butyrolactone (GBL), and the like. Typical examples of the linear chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like.
In order to improve the safety of batteries, various electrolyte additives are developed, and such additives improves the battery safety in overcharge by processes, such as gas generation, oxidation-reduction shuttle reaction and polymerization reaction.
For example, additives which use oxidation-reduction shuttle reaction include chloroanisole and the like. However, they are not effective at a high charge current.
Also, additives which use polymerization reaction include biphenyl, alkylbenzene derivatives, such as cyclohexylbenzene, and the like. These additives block the flow of a current by polymerization reaction in the overcharge condition of batteries. However, the single use of biphenyl has problems in that the battery resistance is increased, the battery performance is deteriorated and biphenyl has to be used in a large amount. Furthermore, the single use of alkylbenzene derivatives, such as cyclohexylbenzene, has problems in that a large amount of additives have to be used, resulting in deterioration in the battery performance.